Skeletal replacement nomenclature

So, in substitutive nomenclature we take a parent hydride or a functional parent, whose name form a root of a term to be created, and replace the hydrogens with substituents, whose names become prefixes or suffixes. Can we do the same with other atoms apart from hydrogens, for the naming purposes?

Yes we can.

(a)	(b)

pentaethylene glycol (trivial) 3,6,9,12-tetraoxatetradecane-1,14-diol (replacement + substitutive) tetradecane

Look at the structure of pentaethylene glycol (a). Its IUPAC name is 3,6,9,12-tetraoxatetradecane-1,14-diol. What’s going on here? We start with the parent hydride, tetradecane (b) and substitute two hydrogens at the C1 and C14 atoms with hydroxy groups, hence “1,14-diol” bit. But we also replace the carbon atoms at the positions 3, 6, 9 and 12 with oxygen atoms, thus “3,6,9,12-tetraoxa” prefix.

The latter naming method is called skeletal replacement nomenclature also known as ‘a’ nomenclature, where ‘a’ refers to the last letter of prefixes used in it [1, p. 337]:

B	→	bora	C	→	carba	N	→	aza	O	→	oxa
Al	→	alumina	Si	→	sila	P	→	phospha	S	→	thia
Ga	→	galla	Ge	→	germa	As	→	arsa	Se	→	selena
In	→	inda	Sn	→	stanna	Sb	→	stiba	Te	→	tellura
Tl	→	thalla	Pb	→	plumba	Bi	→	bisma	Po	→	polona

In my view, there is no fundamental difference between substitution and replacement: after all, the two words mean the same in English. However, the names constructed using these operations are very different.

Going back from names to structures, we have to keep hydrogen accountancy. This is because the “skeletal replacement of a carbon atom” actually means the replacement of a carbon atom plus a number (from zero to three) of implicit hydrogens with another atom plus some implicit hydrogens. In our example, we replaced four –CH₂– groups in (b) with four –O– groups and substituted two hydrogens with two –OH groups to get the structure (a), so we lost eight hydrogens. If we had ‘aza’ replacement instead, we would change –CH₂– groups to –NH– groups. But if we had ‘sila’ replacement, we wouldn’t lose any hydrogens. The number of implicit hydrogens depends on implicit valences of the elements; in the groups 13, 14, 15 and 16 the valences are assumed to be 3, 4, 3 and 2, respectively.

There are some differences between acyclic and cyclic structures as far as naming is concerned.

If our parent structure is an unbranched-chain hydrocarbon, the terminal carbon atoms can also be replaced, but only by B, Al, Ga, In, Tl, Si, Ge, Sn, Pb, P, As, Sb or Bi, not by any other element [2, p. xxix]. So we can’t name the structure (a) 1,4,7,10,13,16-hexaoxahexadecane.

(c)	(d)

tetraethylene glycol (trivial) 2,2′-[oxybis(ethane-2,1-diyloxy)]diethanol (substitutive) undecane

In addition, we need to replace at least four carbon atoms by heteroatoms to use the ‘a’ nomenclature, while “heteroatoms must not belong to the principal characteristic group (if there is one) when counting them for this purpose” [1, p. 94].

Consider tetraethylene glycol (c), a shorter analogue of pentaethylene glycol (a). Logically, we should be able to derive (c) from undecane (d) and name it 3,6,9-trioxaundecane-1,11-diol — but, as we only need to substitute three carbon atoms, we can’t do it. Instead, IUPAC recommends an unwieldy substitutive name, 2,2′-[oxybis(ethane-2,1-diyloxy)]diethanol.

With cycles, there are less restrictions. The ‘a’ nomenclature allows to name structures containing no carbon atoms at all, although that probably will make an inorganic chemist cringe:

(e)	(f)	(g)

C6H12 cyclohexane B3H3O3 boroxin (trivial) 1,3,5-trioxa-2,4,6-triboracyclohexane (replacement) N6 hexazine (Hantzsch-Widman) hexaazacyclohexa-1,3,5-triene (replacement)

Here, all carbon atoms in the cyclohexane (e) skeleton are replaced by other elements. Thus, we can name the structure (f) 1,3,5-trioxa-2,4,6-triboracyclohexane. (Note that the –CH₂– groups are replaced with –BH– and –O– groups.) As for hexazine (g), it can be seen as a result of replacement of all carbons of cyclohexa-1,3,5-triene (aka benzene) with nitrogens, so we can name it hexaazacyclohexa-1,3,5-triene. (So the –CH= groups are replaced with –N= groups.)

The opposite situation is found in the special case of carboranes, which are derived from boron hydrides by the replacement of boron atom(s) with carbon:

(h)	(i)

B12H12 closo-dodecaborane(12) B10C2H12 closo-1,12-dicarbadodecaborane(12) (replacement)

Here, purely inorganic parent hydride (h) closo-dodecaborane(12) undergoes the skeletal replacement resulting in the structure (i) named 1,12-dicarba-closo-dodecaborane(12) or, according to the latest IUPAC recommendations [3], closo-1,12-dicarbadodecaborane(12).

References

1. Connelly, N.G., Hartshorn R.M., Damhus, T. and Hutton, A.T. Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005. Royal Society of Chemistry, Cambridge, 2005.
2. Favre, H.A. and Powell, W.H. Nomenclature of Organic Chemistry: IUPAC Recommendations 2013 and Preferred IUPAC Names. Royal Society of Chemistry, Cambridge, 2014.
3. Beckett, M.A., Brellochs, B., Chizhevsky, I.T., Damhus, T., Hellwich, K.-H., Kennedy, J.D., Laitinen, R., Powell, W.H., Rabinovich, D., Viñas, C. and Yerin, A. (2020) Nomenclature for boranes and related species (IUPAC Recommendations 2019). Pure and Applied Chemistry 92, 355—381.